The present invention relates to emergency lighting systems, and, more particularly, to such systems permitting the use of existing wiring within the building to provide DC power to the units in such systems.
Emergency lighting systems are widely employed to illuminate buildings and to identify exits during interruptions of power supply. In many buildings (particularly of high rise construction), there is a central DC power supply consisting of a 120V DC charger and battery bank. A number of incandescent lighting heads are located throughout the building to provide an emergency lighting system to illuminate pathways for exit purposes when there is a power failure.
Some exit signs contain batteries to operate the lamp in the event of failure of the AC power which is powering the lamp. The AC power supply is charging the battery to keep it up to strength, but, over a period of time, the battery may deteriorate and render the unit ineffective in the event of failure. Moreover, decaying batteries will occasionally leak so as to cause deterioration of the entire sign unit. Thus, there has been a trend to power the exit signs from a central DC power source in the event of power failure or continuously.
In many large installations, there are two separate pairs of wires feeding lighting units in the emerging lighting system. AC power is normally supplied to the lamps of the exit signs through one pair of wires. DC power can be supplied to the DC lamps of the exit signs through the second set of wires from a battery bank and the emergency lighting heads are also supplied power thereby.
So long as there is AC power, a pair of contacts are normally closed in the AC circuit to provide power to the AC pair, and, in the event of a power failure, these contacts open and a pair of contacts in the DC circuit are closed to supply DC power through the DC line. Normally, the AC power source is also charging the battery bank. Such a system is illustrated in FIG. 1.
Although such a pair of two wire circuits may be readily installed in new construction, it is far more difficult to do so in existing buildings without incurring great cost. This type of installation increases the cost of a system because of the need to have a second set of wires to carry the different power supplies. This requirement increases the cost of new construction but presents a far more significant problem in existing buildings because conduits must be run throughout the building. Such conduits typically can be installed only with extensive drilling through concrete floors and walls with attendant high cost. In addition, exposed conduits are aesthetically displeasing and often unacceptable. A second problem is that many exit signs utilize fluorescent lamps which will not operate with an emergency DC power supply, and therefore require installation of a set of exit signs that can operate under AC and DC power. Accordingly, it is an object of the present invention to provide a novel emergency lighting system that enables the use of a single pair of wires to provide both AC and DC power to the light units and thereby reduce the cost and aesthetic impact of providing a separate DC power system.
It is also an object to provide such a system which will accommodate both fluorescent and incandescent lamp loads.
Another object is to provide such a system which will operate the heads of emergency light units only in the event of power failure.
A further object is to provide a novel relay for use in such system to switch between AC and DC.